The prior art discloses electro-acoustic apparatus wherein the associated transducer is located in a housing and the sound output or sound input is coupled through suitable acoustic ducts or tubing to the hearing canal or source of sound. One common application is as components of headsets worn by telephone operators. Another application is as transducers, that is as microphones and receivers in hearing aids wherein the transducer is located within the hearing aid housing. In the application of the transducer as a hearing aid receiver, an acoustic passage from the receiver extends through part of the hearing aid and connects to a length of flexible tubing which couples into an ear mold that substantially seals to a portion of the hearing aid canal. The ear mold creates an effectively closed chamber to couple the sound from the receiver to the ear drum. Also a duct may be used to couple sound from an appropriate operative location to the microphone.
The sound passage, usually comprising a duct or channel, can be of a single length of tubing, but commonly the duct comprises several pieces of tubing which pieces may be of different areas and which may be joined to provide the necessary length of duct.
For example, a common length of ducts used in hearing aids to form the sound channel from the receiver to the chamber adjacent the ear is of the order of two to three inches. Tubing of the foregoing length possesses resonant modes within the frequency range of the normal hearing and these resonances create varying impedance and transmission parameters than can interact with the impedance and transmission parameters of the receiver to provide a fluctuating transmission characteristic to the hearing canal.
Damping elements or materials have previously been added to prior art systems to dampen the amplitude of these fluctuations. There are several places where the insertion of a damping element into the acoustic path has been considered to be functionally acceptable. One such location is in, or adjacent, the receiver wherein the damping elements may actually form a part of the receiver. Another location applicable in a behind-the-ear hearing aid is where the flexible tubing is attached. More specifically, in a receiver for such a behind-the-ear hearing aid application, the most effective point at which to install the damping element is at the end of the duct where it connects to the chamber adjacent the ear drum. At this latter point, a damping element of proper impedance can produce a smooth transmission characteristic. Likewise, when a damping element is provided for a microphone, a damping element is conveniently located adjacent the sound input to the duct. However, locating the damping element for the receiver system at the end of the channel adjacent the ear drum chamber; or, locating the damping element at the sound input of the microphone system develops practical problems, particularly for the reasons which will now be discussed.
Damping elements to be useful must consist principally of acoustic resistance. Known methods of obtaining resistance in acoustic paths (in the frequency range at which the earphone system is to function) comprise the provision of small passages or holes to assure that the resistance or dissipative component of the total impedance is more effective than the inertance component of the impedance. However, when the damping element which comprises small passages is used with a receiver in a hearing aid, the passages are readily stuffed or clogged by the excretions normally present in the ear; or, when a similar damping element is used with a microphone such as with a headset, the damping element may become contaminated or plugged up by moisture and dust. Accordingly, designers have heretofore attempted a compromise between locating the damping elements at a point where the element is most effective, but very apt to malfunction, or locating the damping element at a point where it is partially effective and reasonably free of malfunction.
Accordingly, it is a principal object of the present invention to provide an acoustic transmission system wherein the damping elements may be positioned, where there is minimal likelihood of malfunctioning, and still be of maximum effectiveness.
It is another object of the present invention to provide an acoustic transmission system having improved means for reducing the amplitude of fluctuations of the transmission characteristics within the operating frequency range.
It is another object of the present invention to provide an acoustic transmission system which enables the location of the damping elements at a point other than at the end of an acoustical duct, but which damping elements obtain a high degree of effectiveness.
The foregoing and other features and objects of the present invention will be apparent from the following more particular description of the preferred embodiments of the invention as illustrated in the accompanying drawings wherein: